1. Field of Invention
The present invention relates to: an optical disc recording method in which information is recorded on an optical disc by irradiating the optical disc with an intensity-modulated pulsed laser beam; and an optical disc recorder using the method.
2. Related Art
In recording information on a recordable optical disc, the optical disc is irradiated with an intensity-modulated pulsed laser beam. In this way, the state of a recording film is changed, so that marks and portions (spaces) between the marks are formed. Among recordable optical discs, DVD-Rs and DVD+Rs are well known as write-once read-many (WORM) optical discs on which information can be recorded only once, while DVD-RAMs, DVD-RWs and DVD+RWs are well known as rewritable optical discs on which information can be overwritten. Additionally, in recent years, Blu-ray Discs (BDs), which are large-capacity optical discs using a blue light source, are put into actual use, and are provided as WORM BD-Rs and rewritable BD-REs.
In a mark-edge recording method employed for currently-available optical discs, code information of a mark is specified by front and back edge positions of the mark. Accordingly, in recording information in the method, it is necessary to perform control for appropriately positioning each mark edge by adjusting the power of a laser beam and pulse timing. Edge positions of each recorded mark need to be controlled by taking account of an influence of heat generated in recorded marks before and after the recorded mark as well. For this reason, such control employs an adaptive recording control in which the edge positions of each mark are controlled by classifying times of starting edges of pulses and durations of the pulses on the basis of combination patterns of the length of the mark and lengths of spaces adjacent to the mark.
FIG. 1 is a chart showing recording pulse waveforms in an “N−1 write strategy,” which is a recording method used for BDs and the like. This term “N−1 write strategy” is named after the fact that a mark having a length of NT is recorded by using a pulse string including (N−1) pulses in this recording method. FIG. 1 shows pulse strings for recorded marks having lengths of 2T to 5T (where T indicates a channel bit length) among marks having lengths of 2T to 9T used in BD coding. The initial pulse of a pulse string is called a first pulse, and the last pulse of the pulse string is called a last pulse. Multiple pulses between the first pulse and the last pulse are called multipulses, and the number of the multipulses increases by one as the mark length increases by 1T. It should be noted, however, that the pulse string for a mark having a length of 2T only includes a first pulse, and the pulse string for a mark having a length of 3T includes a first pulse and a last pulse, without including any multipulse. A pulse immediately after the last pulse is called a cooling pulse.
A laser beam can be outputted at output power levels of a write power PW, a space power (erase power) PS, a bias power PBW and a cooling power PC. The write power PW is a power level used for a first pulse, each multipulse and a last pulse, and causes a state change of a recording film by supplying an energy to the recording film. The space power PS is a power level used for irradiating a portion (space) to be positioned between marks. The space power PS is used for preheating for forming a succeeding mark in the case of a WORM disc, and is used for direct rewriting in which a mark is erased and is thereby replaced with a space, in the case of a rewritable disc using a phase-change recording film. The cooling power PC is a power level used for a cooling pulse. The cooling power PC is used to reduce thermal interference by stopping thermal diffusion to a portion in which a succeeding mark is to be recorded, in the case of a WORM disc, and is used to form an amorphous mark by rapid cooling of a recording film after heating, in the case of a rewritable disc. It should be noted that each type of the power levels has the same value irrespective of the mark length.
Parameters related to pulse timing include first-pulse starting-edge time TSFP, a first pulse duration FP, a multipulse duration MP, a last-pulse starting-edge time TSLP, a last pulse duration LP and a cooling pulse duration CP. Here, each of TSFP and TSLP is defined on the basis of an NRZI channel bit signal of recording data, as shown in FIG. 1. Among these pulse parameters, TSFP, FP, TSLP, LP and CP are used to control edge positions of each mark. The value of each of the parameters is set for each combination pattern of the length of a certain mark and the length of a space immediately before the certain mark and for each combination pattern of the length of the certain mark and a space immediately after the certain mark. In the following in this description, a “certain mark” indicates a mark to be subjected to recording pulse control or a mark to be subjected to edge shift detection, a “preceding space” indicates a space immediately before the certain mark, a “succeeding space” indicates a space immediately after the certain mark, and a preceding mark indicates a mark preceding the certain mark.
As a method for adjusting each pulse parameter to an optimal value, known are a method of minimizing pulse jitter and a method of minimizing edge shift. FIG. 2 schematically shows: marks and spaces recorded on a medium; the waveform of an equalized reproduction signal corresponding to the marks and spaces; the waveform of a binarized reproduction signal obtained by binarizing the equalized reproduction signal; and the waveform of a channel bit clock signal generated from the binarized generation signal. Jitter is obtained by normalizing, at a channel clock cycle, a standard deviation of time differences between the binarized reproduction signal and the channel bit clock signal at mark edges. Edge shift is obtained by normalizing, at the channel clock cycle, an average value of time differences between the binarized reproduction signal and the channel bit clock signal at the edges. As to edge shift, the plus symbol indicates a direction in which a light spot moves relative to the optical disc while the minus symbol indicates a direction opposite to the direction indicated by the plus symbol.
FIG. 3 is a flowchart showing an example of a conventional pulse parameter adjustment procedure. In this adjustment procedure, the following write strategy is employed to detect edge shifts. In the write strategy, the parameters TSFP and FP related to front-edge control are classified on the basis of patterns of 4×4 combinations of mark lengths (2T, 3T, 4T and 5T or larger) and preceding space lengths (2T, 3T, 4T and 5T or larger) while the parameters TSLP, LP and CP related to back-edge control are classified on the basis of patterns of 4×4 combinations of mark lengths (2T, 3T, 4T and 5T or larger) and succeeding space lengths (2T, 3T, 4T and 5T or larger). Then, by using the write strategy, edge shifts for front edges are classified and detected on the basis of the combination patterns of the mark lengths and the preceding space lengths; edge shifts for back edges are classified and detected on the basis of the combination patterns of the mark lengths and the succeeding space lengths. Thereby, each of the pulse parameters is adjusted so that the edge shift would be a minimum in each of the patterns.
When the processing is started, values are set for each of the pulse parameters TSFP, FP, MP, TSLP, LP and CP in Step 11. For each of the pulse parameters, predetermined initial values are set in a first loop, and currently-set values are changed in a second or subsequent loop. Here, for each of TSFP and FP related to the front-edge control, values are classified and set on the basis of the combination patterns of the mark lengths and the preceding space lengths; for each of TSLP, LP and CP related to the back-edge control, values are classified and set on the basis of the combination patterns of the mark lengths and the succeeding space lengths. It should be noted that the same value is used for MP for all patterns.
FIGS. 4A and 4B show tables of parameter setting values. FIG. 4A is a table for each of TSFP and FP, while FIG. 4B is a table for each of TSLP, LP and CP. Thus, a total of five tables are used. In FIGS. 4A and 4B, TW denotes a channel bit clock cycle.
In Step 12, random data is recorded on a predetermined portion of an optical disc medium, and the recorded data is reproduced. In Step 13, edge shifts are calculated by using a reproduction signal. Specifically, for front edges, edge shifts are classified and calculated on the basis of the combination patterns of the mark lengths and the preceding space lengths; for back edges, edge shifts are classified and calculated on the basis of the combination patterns of the mark lengths and the succeeding space lengths. FIGS. 5A and 5B each show a table of edge shift values classified on the basis of the patterns. FIG. 5A shows a table obtained by classifying and calculating edge shifts on the basis of the combination patterns of the mark lengths and the preceding space lengths, while FIG. 5B shows a table obtained by classifying and calculating edge shifts on the basis of the combination patterns of the mark lengths and the succeeding space lengths. In Step 14, it is determined whether or not the absolute value of the edge shift obtained on the basis of each of all the patterns is the minimum. If Yes, the processing is terminated. If No, the processing returns to Step 11, and the values of each of the pulse parameters are changed. In this way, the pulse parameters are determined so that the absolute value of the edge shift obtained on the basis of each of all the patterns would be the minimum.
Patent Document 1: JP 2008-108300 A